A six-membered-ring incorporated Si-rhodamine for imaging of copper(ii) in lysosomes

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

A new six-membered spiro-rhodamine probe for Cu2+ and its imaging in mitochondria and lysosomes of Hela cells

Different from five-membered rhodamine spirolactam (Rh-OH), a new six-membered spiro-rhodamine probe (SRh-OH) bearing urea structure has been developed in this paper. Compared with five-membered Rh-OH, six-membered SRh-OH exhibited higher selectivity and sensitivity to Cu²⁺ in aqueous solution. Upon addition of 10 equiv. Cu²⁺, the molar extinction coefficient of SRh-OH at 563 nm can be up to 4.73 × 10⁴ Lmol^-1cm^-1. In the range of Cu²⁺ from 0 to 28 μM, there was an excellent linear relationship between the absorption or emission intensity and Cu²⁺ concentration. The detection limit of SRh-OH was as low as 26 nM (S/N = 3). The reversible binding mode of SRh-OH with Cu²⁺ was further confirmed by S^2- and EDTA addition. Bio-imaging showed SRh-OH can map the distribution of Cu²⁺ not only in mitochondria but also in lysosomes of Hela cells.

Recent development in fluorescent probes for copper ion detection

Abstract:

Copper is the third most common heavy metal and an indispensable component of life. Variations of body copper levels, both structural and cellular, are related to a number of disorders; consequently, pathophysiological importance of copper ions demands the development of sensitivity and selective for detecting these organisms in biological systems. In recent years, the area of fluorescent sensors for detecting copper metal ions has seen revolutionary advances. Consequently, closely related fields have raised awareness of several diseases linked to copper fluctuations. Further developments in this field of analysis could pave the way for new and innovative treatments to combat these diseases. This review reports on recent progress in the advancement of three fields of fluorescent probes; chemodosimeters, near IR fluorescent probes, and ratiometric fluorescent probes. Methods used to develop these fluorescent probes and the mechanisms that govern their reaction to specific analytes and their applications in studying biological systems, are also given.

NIR-emitting styryl dyes with large Stokes' shifts for imaging application: From cellular plasma membrane, mitochondria to Zebrafish neuromast

Near-infrared (NIR) emitting probes with very large Stokes' shifts play a crucial role in bioimaging applications, as the optical signals in this region exhibit high signal to background ratio and allow deeper tissue penetration. Herein we illustrate NIR-emitting probe 2 with very large Stokes' shifts (Δλ ≈ 260 - 272 nm) by integrating the excited-state intramolecular proton transfer (ESIPT) unit 2-(2'-hydroxyphenyl)benzoxazole (HBO) into a pyridinium derived cyanine. The ESIPT not only enhances the Stokes' shifts but also improves the quantum efficiency of the probe 2 (фfl = 0.27 - 0.40 in DCM). The application of 2 in live cells imaging reveals that compound 2 stains mitochondria in eukaryotic cells, normal human lungs fibroblast (NHLF), Zebrafish's neuromast hair cells, and support cells, and inner plasma membrane in prokaryotic cells, Escherichia coli (E. coli).

Therapeutic strategies in Wilson disease: pathophysiology and mode of action

Wilson disease is a copper overload disease treatable with the chelators D-penicillamine and trientine to enhance urinary excretion or with zinc which predominantly inhibits absorption. By lifelong treatment a normal life expectancy and significant improvement of hepatic injury as well as neurologic manifestation is achievable. Here we evaluate the mode of action for effective therapy of Wilson disease. We postulate that there is no quantitative removal of copper from the liver possible. The therapeutic goal is the removal of toxic free copper (non-ceruloplasmin, but albumin bound copper). This is achievable by the induction of metallothionein which is accomplished by chelators and in particular by zinc. For control of therapy the option of a direct measurement of free copper would be preferable over the less reliable calculation of this fraction. A therapeutic challenge is still the full restoration of neurological deficits which can hardly be reached by the available chelators. Whether bis-choline-tetrathiomolybdate as intracellular copper chelator is an option has to be awaited. It is concluded that the goal of actual drug therapy in Wilson disease is the normalization of free copper in serum.

Construction of a dual-response fluorescent probe for copper (II) ions and hydrogen sulfide (H2S) detection in cells and its application in exploring the increased copper-dependent cytotoxicity in present of H2S

Multiple types of metal ions and active small molecules (reactive nitrogen species, reactive oxygen species, reactive sulfur species, etc.) exist in living organisms. They have connections to each other and can interact and/or interfere with each other. To investigate the relationship of metal ions and active small molecules in living cells, it is necessary and critical to develop molecular tools that can track two kinds of associated certain metal ions and reactive molecules with multiple fluorescence signals. However, this is a challenging task that requires an ingenious molecular design to achieve this goal. Here, we present a fluorescent probe (D-CN) that can offer fluorescence imaging of H₂S and copper (II) ions with different response signals. Recognition of H₂S and Cu (II) by the new probe can result in green and red emissions, respectively, providing different signal responses to the two substances in living cells and zebrafish. In addition, we used this probe to visually prove that the cytotoxicity of copper ions in living cells increases in the presence of hydrogen sulfide and could lead to cell apoptosis.

A highly selective fluorescent probe for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate and its application in cell imaging

A highly selective OFF–ON fluorescent probe is developed for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate moiety. The probe is weakly fluorescent due to esterification of the phenolic group. Upon treatment with 1 equiv. of Cu2+, the probe exhibits strong fluorescence at 570 nm. The probe also exhibits high selectivity for Cu2+ over other cations with a low detection limit of 0.2 μM, which is sensitive enough to meet the standard of the World Health Organization for Cu2+ in drinking water (30 μM). Moreover, the probe shows a very low cell cytotoxicity, and imaging experiments demonstrate that the probe can be used for the sensing of Cu2+ in living cells.

Switching of C-C and C-N Coupling/Cleavage for Hypersensitive Detection of Cu2+ by a Catalytically Mediated 2-Aminoimidazolyl-Tailored Six-Membered Rhodamine Probe

A robust six-membered rhodamine spirocyclic probe 1 containing a versatile 2-aminoimidazolyl moiety was elaborately designed and synthesized via an attractive C-C and C-N coupling strategy to improve the performance in the detection of ultralow transition metal ions. Probe 1 allowed the highly hypersensitive detection of Cu²⁺ with a superior picomolar limit of detection (35 pM) and nanomolar naked-eye performance (80 nM) via the switching of C-C and C-N cleavage by a catalytic hydrolysis mode.

Asymmetric Si-rhodamine scaffolds: rational design of pH-durable protease-activated NIR probes in vivo

A group of asymmetric Si-rhodamine scaffolds was designed for protease-activated NIR probes. Dual pH-inertia for both spirocyclized fluorescent probes and fluorescent products of zwitterions form over a wide range of pH (4.0-11.0). Leucine aminopeptidase (LAP) and γ-glutamyl transpeptidase (GGT) were monitored by fluorescent imaging in vivo.

A near-infrared fluorescent probe based on a novel rectilinearly π-extended rhodamine derivative and its applications

A novel NIR fluorescent probe RQNA based on a π-extended rhodamine derivative RQN for the specific detection of mitochondrial Cu²⁺ has been synthesized.

A new approach to silicon rhodamines by Suzuki-Miyaura coupling - scope and limitations

Background: Silicon rhodamines are of particular interest because of their advantageous dye properties (fluorescence- and biostability, quantum efficiency, tolerance to photobleaching). Therefore, silicon rhodamines find frequent application in STED (stimulated emission depletion) microscopy, as sensor molecules for, e.g., ions and as fluorophores for the optical imaging of tumors. Different strategies were already employed for their synthesis. Because of just three known literature examples in which Suzuki–Miyaura cross couplings gave access to silicon rhodamines in poor to moderate yields, we wanted to improve these first valuable experimental results.

Results: The preparation of the xanthene triflate was enhanced and several boron sources were screened to find the optimal coupling partner. After optimization of the palladium catalyst, different substituted boroxines were assessed to explore the scope of the Pd-catalyzed cross-coupling reaction.

Conclusions: A number of silicon rhodamines were synthesized under the optimized conditions in up to 91% yield without the necessity of HPLC purification. Moreover, silicon rhodamines functionalized with free acid moieties are directly accessible in contrast to previously described methods.

A NIR-emitting cyanine with large Stokes shifts for live cell imaging: large impact of the phenol group on emission

There are a limited number of near-infrared (NIR) emitting (λem = 700-900 nm) molecular probes for imaging applications. A NIR-emitting probe that exhibits emission at ∼800 nm with a large Stokes shift was synthesized and found to exhibit excellent selectivity towards mitochondria for live-cell imaging. The photophysical properties were attributed to an excited "cyanine structure" via intramolecular charge transfer (ICT) involving a phenol group.

Synthesis of a dihalogenated pyridinyl silicon rhodamine for mitochondrial imaging by a halogen dance rearrangement

Background: Since their first synthesis, silicon xanthenes and the subsequently developed silicon rhodamines (SiR) gained a lot of attention as attractive fluorescence dyes offering a broad field of application. We aimed for the synthesis of a fluorinable pyridinyl silicon rhodamine for the use in multimodal (PET/OI) medical imaging of mitochondria in cancerous cells.

Results: A dihalogenated fluorinatable pyridinyl rhodamine could be successfully synthesized with the high yield of 85% by application of a halogen dance (HD) rearrangement. The near-infrared dye shows a quantum yield of 0.34, comparable to other organelle targeting SiR derivatives and absorbs at 665 nm (ε_max = 34 000 M⁻¹cm⁻¹) and emits at 681 nm (τ = 1.9 ns). Using colocalization experiments with MitoTracker^® Green FM, we could prove the intrinsic targeting ability to mitochondria in two human cell lines (Pearson coefficient >0.8). The dye is suitable for live cell STED nanoscopy imaging and shows a nontoxic profile which makes it an appropriate candidate for medical imaging.

Conclusions: We present a biocompatible, nontoxic, small molecule near-infrared dye with the option of subsequent radiolabelling and excellent optical properties for medical and bioimaging. As a compound with intrinsic mitochondria targeting ability, the radiolabelled analogue can be applied in multimodal (PET/OI) imaging of mitochondria for diagnostic and therapeutic use in, e.g., cancer patients.

A twist six-membered rhodamine-based fluorescent probe for hypochlorite detection in water and lysosomes of living cells

A novel six-membered rhodamine-based fluorescent probe (6G-ClO) was developed from 2-formyl rhodamine (6G-CHO) and used for hypochlorite detection in water and HUVEC cells. Different from planar penta cycle of rhodamine spirolactam, there was a twist six-membered spirocyclic hydrazone in 6G-ClO optimized by Gaussian software at DFT/B3LYP/6-31G(d) level. The high selectivity, high sensitivity and fast response of 6G-ClO towards ClO^- would be attributed to the twist six-membered spirocycle. Test-strip prepared with 6G-ClO was successfully used to semi-quantitatively indicate the concentration of ClO^- in water. 6G-ClO can also quantitatively detect the concentration of ClO^- in tap water and swimming pool water. The detection limit of 6G-ClO was as low as 12 nM. The co-localization staining of HUVEC cells further verified that 6G-ClO could specifically accumulate in lysosomes and capture exogenous/endogenous ClO^- in living lysosomes. 6G-ClO would be a practical probe for real-time monitoring of ClO^- in the biological and real water samples.

A functional preservation strategy for the production of highly photoluminescent emerald carbon dots for lysosome targeting and lysosomal pH imaging

Lysosomes, which can be easily targeted by molecules with abundant amino groups, play critical roles in endocytosis, autophagy, and phagocytosis; thus, it is important to accurately characterize lysosomes, including lysosomal pH, in living cells to understand their physiological and pathological functions. Herein, a new type of highly photoluminescent (PL) emerald carbon dots (CDs) was easily prepared through a functional preservation strategy (FPS) by simply mixing p-benzoquinone and ethanediamine at room temperature. The as-prepared CDs possessed abundant amino groups preserved from ethanediamine owing to FPS, and they exhibited excellent photostability as compared to the commercial LysoTracker probes. Consequently, they actively targeted lysosomes to sensitively respond to lysosomal pH in vitro owing to their abundant amino groups and good hydrophilicity. Thus, we could successfully monitor lysosomal pH dynamics during apoptosis in live cells.

An NIR-emitting lysosome-targeting probe with large Stokes shift via coupling cyanine and excited-state intramolecular proton transfer

An NIR-emitting probe (λ_em ∼ 700 nm) with a large Stokes shift (Δλ ≈ 234 nm) is synthesized by using excited-state intramolecular proton transfer (ESIPT). The phenolic proton, which controls ESIPT, acts as a switch to give strong fluorescence at pH ≈ 5. The probe can selectively show lysosome organelles, therefore leading to a lysosome probe without exhibiting "an alkalinizing effect".

New Six-Membered pH-Insensitive Rhodamine Spirocycle in Selective Sensing of Cu2+ through C-C Bond Cleavage and Its Application in Cell Imaging

A new rhodamine-based chemosensor 1 with a six-membered spirocyclic ring has been synthesized, which exhibits excellent pH stability and shows selective "turn-on" fluorescent detection of Cu²⁺ ions over a series of other metal ions including Cu⁺ ions. The expansion of spirocycle improves the stability and selectivity of the chemosensors in sensing of metal ions. Till today only few rhodamine structures R1-R5 with thiourea-, hydrazine amide-, or pyrrole-decorated six-membered spirocyclic rings are known that exhibit metal-ion sensing via C-N bond cleavage of the spiro ring. In this context, rhodamine compound that responds to the metal ion through C-C bond cleavage of the six-membered spiro ring is completely unknown. The present example is a first-time report that demonstrates selective sensing of Cu²⁺ ions through C-C bond cleavage over the conventional existing systems in the literature. The chemosensor 1 is cell permeable and can detect Cu²⁺ in live cells using confocal microscopy in the biologically relevant pH range with high photostability.

A reaction-based fluorescent turn-on probe for Cu2+ in complete aqueous solution

FP can detect Cu²⁺ in complete aqueous solution with a rapid response time, high sensitivity, and high selectivity.